The present invention relates to an impregnated cathode used in an electron tube or the like and, more particularly, to a surface coating layer thereof, used for thermionic emission
An impregnated cathode is obtained by impregnating pores of a porous pellet with an electron-emission material such as barium oxide, calcium oxide, aluminum oxide, etc. Such a cathode can provide a current density higher than a conventional oxide thermal cathode, and has a longer service life, since it is resistant to a harmful gas, contained in a tube, and which interferes with electron emission. Consequently, cathodes of this type are employed in a travelling-wave tube used in, for example, artificial satellites, in a high-power klystron used for plasma heating in a nuclear fusion reactor, etc.
In the above fields, high reliability (long service life, stable operation, and so on) and high current density are required of a cathode. As a means of increasing the reliability, a layer of an element of the platinum group, such as iridium, osmium, ruthenium, etc. or an alloy thereof, is coated on the cathode surface, in order to decrease the work function of the cathode surface, thereby to decrease the operating temperature In contrast to a case wherein such a coating layer is not provided, the operating temperature of a cathode having a coating layer can be decreased by several tens to one hundred and several tens .degree.C., to obtain the same current density. Since evaporation of the electron emission material can then be limited, this is advantageous for a cathode, with regard to prolongation of its service life, and provides an improvement in the intratube withstand voltage characteristics.
However, the operating temperature in this case is still as high as 900.degree. to 1,000.degree. C. Therefore, W for forming a pellet is diffused in the surface coating layer during operation, and forms an alloy together with a metal constituting the surface coating layer. Alloying of the surface coating layer changes the electron-emission characteristics, and interferes with the achieving of stable characteristics from an early stage of operation, and with the prolongation of the service life.